1. Field of the Invention
This invention relates to an exhaust gas purifying system configured to collect carbon particles and so on from exhaust gas of an internal combustion engine, and more particularly to an exhaust gas purifying system which oxidizes and burns carbon particles trapped on a filter using nitrogen dioxide (NO2) generated by an oxidation catalyst.
2. Description of the Related Art
Particulates composed of carbon particles and so on get mixed in exhaust gas of an internal combustion engine, e.g. a diesel engine. A particulate filter is installed in an exhaust passage in order to trap particulates and prevent them from being discharged into the air. When more particulates are accumulated on the particulate filter, they should be burnt in order to regenerate the particulate filter.
In order to overcome the foregoing problem, forced regenerating units are used, which heat the particulate filter and burn particulates when an amount of accumulated particulates exceeds a regeneration reference point. Specifically, the amount of accumulated particulates is detected on the basis of relationship between the flow of exhaust gas and pressure loss of the particulate filter. For instance, some forced regenerating unit injects fuel to a fuel supply system during an expansion stroke or an exhaust stroke after the main fuel injection, and forcibly raises the temperature of exhaust gas. In another example, an electric heater or a gas oil burner is operated to forcibly heat exhaust gas.
The foregoing forced regenerating units tend to reduce the fuel efficiency since particulate filters should be kept hot. In order to overcome this problem, it is necessary to precisely detect forced regeneration timings and lengthen forced regeneration intervals.
Usually, particulates can be oxidized at approximately 600° C. There is a continuous regeneration type filter in which particulates can be burnt at a low temperature of approximately 250° C. This enables particulates to be burnt in a wide temperature range and promotes regeneration of particulate filters.
In the continuous regeneration type filter, an oxidation catalyst is disposed upstream of a particulate filter in an exhaust passage. The oxidation catalyst oxidizes nitric monoxide (NO) and generates nitrogen dioxide (NO2) as expressed by the formula (1).2NO+O2→2NO2  (1)
Nitrogen dioxide (NO2) is very active, and promotes the reaction expressed by formulas (2) and (3) when it comes into contact with particulates trapped on the particulate filter, thereby regenerating the particulate filter.NO2+C→NO+CO  (2)NO2+CO→NO+CO2  (3)
However, the continuous regeneration type filter which can burn particulates at the low temperature fails to raise the temperature of exhaust gas when a vehicle keeps on cruising through town at a low load. In such a case, particulates easily accumulate on the particulate filter, and should be forcibly burnt in order to regenerate the particulate filter.
Therefore, the foregoing continuous regeneration type filter usually includes a forced regeneration unit, which forcibly heats exhaust gas on the particulate filter, and burns particulates when the amount of accumulated particulates is detected to be above the regeneration reference point mentioned above. In this case, the forced regenerating unit injects fuel to a fuel supply system during the expansion stroke or exhaust stroke after the main fuel injection, and forcibly heats exhaust gas.
For instance, the assignee of this application has proposed a method of easily estimating an amount of particulates accumulated on a filter on the basis of an exhaust gas temperature frequency (i.e. at which an exhaust gas temperature is equal to or higher than a predetermined value) as disclosed in Japanese Patent Application No. 2001-144,501 (called the “cited reference 1”). Further, Japanese Patent Laid-Open Publication No. 2002-276,422 (called the “cited reference 2”) discloses a continuous regeneration type DPF (diesel particulate filter) in which an oxidation catalyst, a particulate filter and a NOx catalyst are arranged upstream of an exhaust passage in order to operate an engine by increasing an air-to-fuel ratio during the regeneration of the particulate filter.
In either the continuous regeneration type filter or a simple particulate filter, particulates are burnt when the amount of accumulated particulates exceeds a regeneration reference point. If the amount of accumulated particulates is not precisely detected, e.g. if the accumulated amount is recognized to be excessive, regeneration intervals may be shortened, which reduces fuel efficiency. On the contrary, if the accumulated amount is determined to be small, particulates excessively accumulate on the filter, and may damage the filter when burnt. Therefore, it is necessary to precisely detect forced regeneration timings and to lengthen forced regeneration intervals.
The foregoing method allows detection of the amount of accumulated particulates on the basis of the relationship between the flow of exhaust gas and pressure loss of the filter. However, there is a strong demand for a method of precisely estimating an amount of accumulated particulates. Especially, in the case of the continuous regeneration type filter, particulates tend to be partially burnt, which would lead to non-uniform accumulation of particulates, and further adversely affect relationship between the flow rate of exhaust gas, the pressure loss and the amount of accumulated particulates.
The continuous regeneration type filter (of the cited reference 1) is preferably to be improved. This is because the amount of burnt particulates can be estimated while the amount of discharged particulates cannot be accurately estimated, which would adversely affect precise detection of the amount of accumulated particulates. In the continuous regeneration type filter (of the cited reference 2), the timing to regenerate the particulate filter is not determined on the basis of the amount of accumulated particulates, but the particulate filter is regenerated only by increasing the air-to-fuel ratio, which tends to reduce fuel efficiency.
The present invention is intended to provide an exhaust gas purifying system for an internal combustion engine which can precisely detect a forced regeneration timing, lengthen regeneration intervals, and prevent reduction of fuel efficiency.